Ask HN: Why don't publishers with ad networks route ads through content domains?

In the eternal battle between ad networks and ad blockers, I'm wondering why ad networks who are also publishers haven't taken the obvious route of hosting ads on their publishing domain? This would make it significantly less easy for me to block their third party script domains.

Specifically, why doesn't YouTube put their video ads on the same domain as the videos themselves, or even embed them dynamically in the videos at request time? Why doesn't Twitch stitch their ads into the live video stream?

I know for third party sites this isn't possible, because ad networks don't trust them to report honestly and want their own scripts running. But for the sites mentioned, and others like them, I haven't been able to figure out their reasoning for not doing this.